San Anselmo Flood Risk Reduction Project (Formerly the Memorial Park Dual-Use Detention Basin Project)


  • Curious what other flood diversion storage basins look like that are similar to what is under consideration at the Sunnyside Nursery site? Take a short visual tour (pdf) of two public parks and playfields in Windsor and Napa. In December 2014, the Windsor park helped prevent flooding when it partially filled for 6 hours and then drained back into the creek.

About the Project

The San Anselmo Flood Risk Reduction Project will reduce the risk of flooding in Ross Valley with the improved former Nursery site providing much-needed, temporary water storage during infrequent, but severe, flood events such as the ones experienced in 1982 and 2005 that caused tens of millions of dollars of damage to private and public property in Fairfax and the lower Ross Valley. The limited in-channel capacity of Sleepy Hollow and San Anselmo Creeks causes flooding in San Anselmo during flood events, and the improved former Nursery would provide temporary water storage, thereby reducing creek overflow and downstream flooding. The Project Environmental Impact Report will analyze flood diversion and temporary storage basin at the former Nursery site, coupled with removal or modification of a commercial building over San Anselmo Creek and creek improvements along Creek Park and downstream, along with three other alternatives and the “no project” alternative. This Project replaces the former Memorial Park Detention Basin project and retains the grant funding authorized for Memorial Park to fund similar flood protection projects within the Ross Valley.

Status & Next Steps

The project is currently in the Environmental Review phase.  A Project Environmental Impact Review Scoping Meeting is scheduled for the evening of April 20, 2017 at San Anselmo Town Hall to collect public comment on potential environmental effects. More specific meeting details will soon be available. District Staff is also working on a scope of work for the engineering feasibility to support the CEQA analysis.

Community-driven process to develop refined concepts:

Project design would begin as part of a community-driven process with multiple opportunities for public participation and input on different design alternatives. Design alternatives would be developed to:

  • maintain the quality of adjoining neighborhoods;
  • ensure basin  design meets community needs;
  • ensure compliance with local, state, and federal environmental laws and regulations;
  • provide opportunities for consistent public participation & input at key decision points; and,
  • protect the public's health and safety.

A preferred design alternative would then be finalized for possible construction. A preliminary schedule and list of known upcoming meetings is provided below.

Preliminary Schedule (Pending Funding & Project Approval)

Project Phase

Typical Duration

Initial Feasibility Assessment

6-9 months

Environmental Review

9-10 months

Selection of Preferred Alternative

1-3 months

Design Development & Final Design

18-24 months

Construction of Preferred Alternative

18-24 months

Total Project Duration

3-4 Years

Upcoming Public Meetings

May 3, 2017: The Town of San Anselmo is hosting a community meeting from 7-8:30p at the San Anselmo Town Council Chambers to discuss potential plans for the building at 634-636 San Anselmo Ave (Building Bridge 2) and flood mitigation related to the San Anselmo Flood Risk Reduction Project.

Past Public Meetings

April 20, 2017: Approximately 30 community members gathered on Thursday evening at San Anselmo Town Hall in San Anselmo for the EIR Public Scoping meeting. The presentation can be accessed by clicking here

The Notice of Preparation (NOP) can be viewed by clicking here. Written comments about the scope of the environmental report were accepted until the close of the NOP comment period which ended on May 8, 2017.

January 24, 2016: Approximately 20 community members gathered on Thursday evening at San Anselmo Town Hall in San Anselmo to learn about the alternative project options for the Memorial Grant funds and to learn how the potential projects and hear help reduce flooding. Attendees asked several questions, shared ideas & concerns, and provided feedback to the council and staff. View the meeting presentation, minutes & comments, and video.

Background Documents

Some of these documents and concept plans may be outdated and superceded. Please contact the project manager to confirm the most current plans for this project.

April 20, 2017 presentation for the project EIR scoping meeting and Notice of Preparation for the project EIR.

March 21, 2017 presentation
to update the Advisory Board on the status of CEQA review

January 24, 2017 presentation to San Anselmo Town Council

December 12, 2016 presentation to the Advisory Board for CEQA review

June 21, 2016 presentation to the Advisory Board on the Memorial Park Grant Migration

April 14, 2016 Technical Memorandum on the Analysis of Project Alternatives for the Memorial Park DWR Grant

Naturalist Basin concept plan designed by Matt Smeltzer

Final Engineering Analysis and Conceptual Design Report and Update of Piezometer Data Memorandum by URS. April 17, 2015. This document includes geotechnical investigations, sediment loading calculations, and an updated plan and cost estimate.

Draft Conceptual Plan View by URS. April 23, 2014. This document was also attached to the November 2015 Flow Reduction Study (35.9MB) in Appendix B.

Memorial Park Detention Basin Project DWR Grant Application - All sections below submitted to CA Department of Water Resources for Prop 1E Stormwater Flood Management Grant - Round 2 submitted February 1, 2013.

2011 Technical Memorandum No. 2 on Detention Basins - Capital Improvement Plan Study. Dated January 27, 2011:

Cost & Funding

The project is preliminarily estimated to cost $17.4 million. Cost estimates will be refined with modeling and through the design process.  Further project development and feasibility analysis may result in increased estimated cost. Identified funding sources include the Department of Water Resources Grant, Ross Valley Flood Control Zone 9 Storm Drainage Fee and other potential grant funding sources.

Implementing Agency & Partners

Marin County Flood Control & Water Conservation District in cooperation with the Town of San Anselmo and the Town of Fairfax.


Liz Lewis, Planning Manager ( or Russ Eberwein, P.E., Senior Civil Engineer ( manager for the Marin County Flood Control & Water Conservation District.